Milkweed flowers - Asclepias species produce some of the most complex flowers in the plant kingdom, comparable to orchids in complexity. 

— Wikipedia

Monarch caterpillars only eat milkweed. If monarch eggs are laid on plants other than milkweed, the caterpillars cannot survive and ultimately starve to death. Monarch caterpillars rely on milkweed plants and that’s why female monarchs choose to lay their eggs on milkweed plants.